The present invention relates to systems and methods for treatment of metal scrap contaminated with combustible substances, and more particularly to apparatus and method for efficiently and automatically insuring complete combustion of contaminants removed from the scrap.
The reclamation of metal scrap by melting requires some preliminary treatment for removal of contaminants on the scrap, such as oil, grease and other similar organic contaminants. Metal scrap should be understood to include swarf, turnings, chips and other materials generated during metal wording operations with metals such as cast iron, aluminum, aluminum alloys, magnesium, and magnesium alloys and others.
The preparatory treatment is usually referred to in the art as drying and is often conveniently carried out in a drying apparatus, one embodiment of which includes an elongated rotating drum slightly sloping with respect to the horizontal. Examples of scrap dryers are shown in U.S. Pat. Nos. 3,619,907; 3,619,908; and 3,767,179.
Typically, scrap is introduced into such a rotating drum at one end, for example, a relatively higher input end, and upon rotation of the drum the scrap travels downwardly towards the lower or output end. During this travel the scrap contacts hot gases which are generated by a burner situated at one end of the drum. The gases are sufficiently hot to effect evaporation of such contaminants and to bring about at least a partial combustion thereof. If a sufficient amount of combustible contaminants are present, a self-supporting flame can be maintained within the drying mechanism.
In order to provide complete combustion of the waste material, exhaust gases which typically contain combustible material are directed into an afterburner device. The temperature in the afterburner is desirably controlled to insure a temperature high enough to complete all combustion without maintaining an excessively high temperature. Typically, the temperature in the afterburner is controlled by regulating one or more burners or heaters utilized to heat the afterburner to maintain the desired temperature. A secondary supply of air may be provided to insure adequate oxygen for complete combustion.
The temperature in the afterburner is not only affected by burner operation, but also by the temperature of the exhaust from the dryer and by the existence or absence of unburned combustibles in this exhaust.
Thus, when the exhaust contains smaller amounts of unburned combustibles, the burners must be selectively energized to maintain the desired afterburner temperature. When the exhaust contains unburned combustible wastes, the combustion of these wastes in the afterburner may be sufficient to maintain temperature in the afterburner without any meaningful additional heat supplied by heaters or burners.
In addition, in order to prevent overheating in the afterburner, the secondary external air supply is also controllable to supply additional air to lower the temperature in the afterburner, if necessary.
Typically, in order to provide sufficient capacity for maintaining desired temperature in the absence of unburned combustibles, it is usually necessary to utilize a plurality of afterburner heaters. In practice, the burners utilized in the afterburner section of the dryer typically have a maximum heat producing ratio of 3:1 between the maximum and minimum settings. Thus, even when the temperature in the afterburner requires no additional heat input, the burners are producing heat and as a result there is a requirement for additional secondary air in order to prevent the temperature in the afterburner from exceeding the maximum limits.
It can be readily appreciated that this type of operation in which the minimum heat output is 1/3 of the total maximum firing rate and heat output of all the burners results in considerable inefficiencies, and more importantly in these days of energy conservation, in waste of fuel.